Updated Photometry of the Yellow Supergiant Progenitor and Late-time Observations of the Type IIb Supernova 2016gkg

TL;DR

This paper presents late-time HST observations of SN 2016gkg confirming the disappearance of its progenitor, analyzes its light curve and spectra showing a shift to circumstellar interaction, and revisits progenitor models considering binary evolution.

Contribution

It provides new late-time imaging confirming the progenitor's disappearance and offers insights into the supernova's evolution and progenitor system through spectral and light curve analysis.

Findings

Progenitor star confirmed to have disappeared.

SN 2016gkg's decline rate is slower than ${}^{56}$Co decay.

Supernova is now powered mainly by circumstellar interaction.

Abstract

We present Hubble Space Telescope (HST) observations of the type IIb supernova (SN) 2016gkg at 652, 1698, and 1795 days with the Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3). Comparing to pre-explosion imaging from 2001 obtained with the Wide Field Planetary Camera 2, we demonstrate that SN 2016gkg is now fainter than its candidate counterpart in the latest WFC3 imaging, implying that the counterpart has disappeared and confirming that it was the SN progenitor star. We show the latest light curve and Keck spectra of SN 2016gkg, which implies that SN 2016gkg is declining more slowly than the expected rate for ${}^{56}$Co decay during its nebular phase. We find that this emission is too luminous to be powered by other radioisotopes, thus we infer that SN 2016gkg is entering a new phase in its evolution where it is powered primarily by interaction with circumstellar matter. Finally, we re-analyze the progenitor star spectral energy distribution and late-time limits in the context of binary evolution models and including emission from a potential companion star and find that all companion stars would be fainter than our limiting magnitudes.